1. Field of the Invention
This invention relates generally to a process for controlling the relative humidity of membranes in a fuel cell stack during system shut-down and, more particularly, to a process for providing an optimal purge that causes membranes in a fuel cell stack to rapidly reach desired hydration limits of the membranes.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
As is well understood in the art, fuel cell membranes operate with a certain relative humidity (RH) so that the ionic resistance across the membrane is low enough to effectively conduct protons. The relative humidity of the cathode outlet gas from the fuel cell stack has a strong influence over the membrane relative humidity. By holding a particular set-point for cathode outlet relative humidity, typically 80%, the proper stack membrane relative humidity can be maintained. Stack pressure, stack temperature, cathode stoichiometry and relative humidity of the cathode air into the stack are all controlled parameters to maintain relative humidity at the air outlet. For stack durability purposes, it is desirable to minimize the number of relative humidity cycles of the membrane because cycling between RH extremes has been shown to severely limit membrane life. Particularly, membrane RH cycling causes the membrane to expand and contract as a result of the absorption of water and subsequent drying. This expansion and contraction of the membrane causes pin holes in the membrane, which create hydrogen and oxygen cross-over through the membrane creating hot spots that further increase the size of the hole in the membrane, thus reducing its life.
As mentioned above, water is generated as a by-product of the stack operation. Therefore, the cathode exhaust gas from the stack will include water vapor and liquid water. It is known in the art to recover water from the cathode exhaust stream and return it to the stack via the cathode inlet airflow. Many devices could be used to perform this function, such as a water vapor transfer (WVT) unit.
Further, when the power request for the stack increases, the compressor speed increases to provide the proper amount of cathode air for the requested power. However, when the compressor speed increases, the flow of air through the WVT unit has a higher speed, and less of a chance of being humidified to the desired level. Also, in some fuel cell system designs, the relative humidity of the cathode inlet stream and/or the cathode exhaust stream can be controlled to maintain a set-point by adjusting the temperature of the cooling fluid flow.
During fuel cell system shut-down, it is desirable that the membranes have a certain relative humidity so they are not too wet or too dry. A membrane that is too wet may cause problems for low temperature environments where freezing of the water in the fuel cell stack could produce ice that blocks flow channels and affects the restart of the system. Therefore, it is known in the art to purge the flow channels in the fuel cell stack, typically using compressor air from the compressor to purge one or both of the cathode and anode of the stack. However, too long of a purge could cause the membranes to become too dry where the membranes will have too low of an electrical conductivity at the next system restart that affects restart performance as well as reduces the durability of the stack. Furthermore, it is desirable to purge the fuel cell system such that the membranes reach the desired amount of relative humidity quickly.